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Introductory Electromagnetism Experimental Laboratory
Mirrors
Goals: Observe the laws of reflection in a plane mirror. Measure a variable that should be consistent with zero.
APPARATUS A plane mirror is a highly reflective flat surface usually coated by a sheet of glass. Beacuse of the glass the reflective surface is actually closer to the back side of the mir- ror than to the front side.
This experiment uses a plane mirror that can be set vertically on stands and long straight pins that can be stuck vertically through a piece of paper on a piece of cardboard. One of those pins will be iused as the object pin infront of the mirror. Other pins will be used as image pins either in front or behind the mirror depending on the experiment. Figure 1 shows the relative positions of the mirror and pins shown from above.
FIGURE 1. Mirror, object pin and image pins.
cardboard and paper
object pin
image pin
image pins
(first exp.)
(second exp.)
mirror
2 Mirrors
In this laboratory the image pins are lined up with the image of the object pin. It is important to keep the pins as vertical as possible. When viewing the pins and the mirror the line of sight should be at about the same height as the pins and mirror.
THEORY Light from an object can be traced as rays. A ray is line that connects the object to the observer and represents a path that light follows between those two points. Rays can be reflected or refracted (bent) when they strike certain surfaces. When rays from an object converge at another point a real image is formed. A real image can be viewed on a screen.
When rays are reflected or refracted to an observer the observer expects the rays to emerge from a single source in straight lines. If the rays seen by the observer trace back to a point not on the actual rays, there is a virtual image. The reflection in a plane mirror is an example of a virtual image since the rays reflected off the mirror trace back to a point behind the mirror, not to the object.
FIGURE 2. Law of reflection.
The law of reflection states that the angle of incidence θ i is equal to the angle of reflec- tion θ r.
(EQ 1)
From that law and some geometry it can be shown the object distance d (^) o to the mirror equals the image distance d (^) i to the mirror.
(EQ 2)
Both EQ 1 and EQ 2 have been written as differences instead of a simple equality. When two measured values are expected to be equal, it is often useful to measure the differ- ence between the two values. By measuring the difference it is possible that some mea- surement uncertainty that appear in the two separate measurements cancels out. The measured difference can be compared to zero to see how well the equality holds.
object observer
mirror
image
do θ i θ r
di
Δθ = θ r – θ i = 0
Δ d = di – d (^) o = 0
4 Mirrors
OBSERVATIONS For each of these questions make your observation and support it by answering the question “Why?”.
How well do you think the line drawn for the back of the mirror in step 1 represents the actual position? Quantify this if possible.
How well did the marks for the pins represent their actual position? Quantify this if pos- sible.
How accurately could the protractor measure angles?
According to your data in steps 16 and 17 does the law of reflection hold for the angles? Explain in terms of your answers for the line positions and protractor measurement.
According to your data in steps 21 and 22 does the law of reflection hold for the object and image distances? Explain in terms of the measurements involved.
